SIP ALG is controlled per device attached to the router, it’s not a setting for the router as a whole.
- Log into your router
- Navigate to: Settings>Advanced Settings>Business Network>Devices
- Select a device by clicking on it
- Change ‘Disable SIP ALG’ to ‘Yes’
- Hit ‘Apply’
This applies to the BT Business Hub 3 (BH3) and possibly also the Home Hub 3 (HH3).